A known effective means against interference light or interfering light points is the use of a mechanical shutter for shutting out such interfering light or interfering light points from the reticle. Thus, it is possible to make sure that only the intensity of the remaining image point of the received light is effective on the detector. However, it is a disadvantage of all methods employing a shutter that interfering processes are not recognized and hence that they cannot be pursued. As soon as a brighter process or interference occurs than the signal process (i.e. the IR-signature of the missile) in the shutter window, the interference overshadows the signal process. This disadvantage lead to the conception of the so-called fixed shutter method, wherein shortly after the start of the missile a mechanical initially large aperture shutter is placed in front of the reticle. Thereafter, as a function of time ever smaller shutters are inserted.
Said fixed shutter method has the disadvantage that the system must work, on the one hand, with relatively large shutter windows so that interference sources in the vicinity of the missile cannot effectively be shuttered out. On the other hand, movements of the missile resulting in larger deviations from the line of sight may cause the loss of the missile by the tracking system.
Such a disadvantage, or rather loss, is avoided by the shutter shifting method (travelling shutter). In that method a mechanical or opto-electronic shutter which is switchable to become transparent, is caused to follow the image point of the received light on the reticle, whereby the size and location of the shutter is variable. The advantage of the shutter shifting method, as compared to the fixed shutter method, is seen in that the course of the motion and intensity of the useful or desired process is followed, whereby it can be continuously compared, for example with a model course. As a result it is possible to recognize a briefly occurring interfering process and to overcome the interfering process by means of a prediction process. However, even the shutter shifting method has the disadvantage that outside of the shutter window no information regarding any interfering process can be obtained and evaluated.
The opto-electronic shutter shifting method in which the shutter is switched to become transparent and which uses a liquid crystal matrix avoids the movable parts that are required in the mechanical shutter shifting method. However, the opto-electronic shutter shifting method has the disadvantage that the transparency cannot be switched between totally open and totally closed so to speak. Rather, an opto-electronically switched shutter is in fact only about 70% "open" in its transparent condition and in its "closed" condition the shutter is only less transparent by a factor of roughly 300 as compared to the open transparency. A mechanical shutter is either fully open or fully closed.